1. Field of the Invention
The present invention relates to an optical heterodyne detection method and an optical heterodyne detecting apparatus for detecting a beat caused by the difference in frequency (referred to as a "beat signal" hereinafter) between two light rays superimposed with each other, or detecting the intensity modulation caused by the difference in phase between the two light rays by using a light receiving element. More particularly, this invention relates to an optical heterodyne detection method and an optical heterodyne detecting apparatus capable of detecting the beat signal (or the intensity modulation signal) regardless of the angular deviation between wavefronts of two light rays, i.e., a signal beam and a reference beam.
2. Description of the Related Art
By utilizing optical characteristics of light such as coherence, multiplexing and high velocity, various kinds of optical instruments such as optical communication apparatus and high accuracy distance measuring instruments have been developed so far. Especially in recent years, a spatial tracking system has been developed as a future optical communication apparatus for realizing a high-speed and large capacity communication between satellites or between a satellite and a space airframe.
In this type of optical communication apparatus, light receiving techniques for receiving a signal beam is important. For example, an optical heterodyne detection method utilizing the coherence of light is one of such light receiving techniques. The practical use of the optical heterodyne detection method was facilitated by the introduction of a laser, which serves as a light source of single-wavelength and superior coherence.
The optical heterodyne detection method used in wireless optical communication will be described hereinafter. Two electric fields formed by two coherent light beams (referred to as a signal beam and a reference beam hereinafter), whose polarizing planes and wavefronts are aligned with each other, are called E.sub.1 and E.sub.2, respectively. Then, the electric field E.sub.1 of the signal beam and the electric field E.sub.2 of the reference beam are represented by the following Formula 1 and Formula 2, respectively: EQU E.sub.1 =A.sub.1 exp [i(2.pi..nu..sub.1 t+.phi..sub.1)] Formula 1 EQU E.sub.2 =A.sub.2 exp [i(2.pi..nu..sub.2 t+.phi..sub.2)] Formula 2
.nu..sub.1 : frequency of signal beam PA0 .nu..sub.2 : frequency of reference beam PA0 .phi..sub.1 : phase of signal beam PA0 .phi..sub.2 : phase of reference beam PA0 A.sub.1 : amplitude of signal beam PA0 A.sub.2 : amplitude of reference beam
An optical current intensity I observed by a light receiving element, to which the signal beam and the reference beam are guided, is obtained by mixing both beams together and then squaring the mixed result for performing an optical detection: EQU I=.vertline.E.sub.1 +E.sub.2 .vertline..sup.2 =A.sub.1.sup.2 +A.sub.2.sup.2 +2A.sub.1 A.sub.2 cos [2.pi.(.nu..sub.1 -.nu..sub.2)t+.phi..sub.1 -.phi..sub.2 ] Formula 3
As is apparent from this Formula 3, the optical current intensity I is composed of the amount of direct current and the beat signal on the frequency of (.nu..sub.1 -.nu..sub.2). Accordingly, it is essential to detect the beat signal in the optical heterodyne detection method.
In general, the optical communication system is so arranged that the signal beam is transmitted from the transmitter side and the reference beam is emitted from the receiver side. In such an arrangement, an optical alignment technique with high accuracy is required for aligning the wavefront of the signal beam with that of the reference beam.
Now, referring to FIG. 15, it is assumed that two wavefronts of a signal beam 206 and a reference beam 202 differ from each other by .delta. degrees, and the optical heterodyne detection is carried out by a light receiving element 200 which includes a light receiving section having a size D. Then, an intensity IF.sub.x of the beat signal at a point x on the light receiving section is as follows: EQU IF.sub.x =2A.sub.1 A.sub.2 cos [2.pi.(.nu..sub.1 -.nu..sub.2)t+.phi..sub.1 -.phi..sub.2 +2.pi.x sin .delta./.lambda..sub.1 ] Formula 4
where .lambda..sub.1 stands for the wavelength of the signal beam 206.
Also, a beat signal intensity IF with respect to the entire light receiving section is as follows: ##EQU1##
As is apparent from this Formula 5, the amplitude of the best signal intensity IF decreases in proportion to sin (.pi.D sin .delta./.lambda..sub.1)/(.pi.D sin .delta./.lambda..sub.1).
FIG. 16 shows the relationship between the light receiving section size D and an upper limit value of the deviation angle .delta. wherein a stable beat signal detection is ensured. This figure shows the case where the wavelength .lambda..sub.1 of the signal beam 206 is 780 nm. As is apparent from FIG. 17, the upper limit value of the deviation angle .delta. allowing a stable beat signal detection depends on the size D of the light receiving section as well as on the wavelength .lambda..sub.1 of the signal beam 206. Stable detection is assured at a deviation angle .delta. where the beat signal is not 0.
FIG. 17 shows the behavior of the relative intensity of the best signal intensity IF where the wavelength .lambda..sub.1 is 780 nm and the size D of the light receiving section is 500 .mu.m. In this figure, the vertical axis represents the relative intensity of the beat signal, and the horizontal axis represents the deviation angle .delta. of wavefronts.
As is shown by FIG. 17, as the wavefront deviation angle .delta. increases, the best signal intensity IF rapidly decreases to 0 at a point where .delta. becomes 0.089 degree. Accordingly, the detection of the beat signal is prevented even by a very small angular deviation of wavefronts of the signal beam 206 and the reference beam 202.
This problem is solved by aligning the wavefronts of the signal beam 206 and the reference beam 202. Conventionally proposed techniques realizing such an alignment are, for example, a spatial tracking system described on p. 125 of the June 1992 issue of "O plus E" or a method of using an optical coupler described on p. 100 of the January 1990 issue of the same publication.
The principle of the spatial tracking system described on the above publication will be described with reference to FIG. 18. This spatial tracking system detects the deviation angle .delta. between the signal beam 206 and the reference beam 202 by a quadrant detecting apparatus 201. Then, the incident angle of the reference beam 202 is mechanically controlled so that the deviation angle .delta. becomes 0, in such a manner that the wavefronts of the reference beam 202 and the signal beam 206 are aligned with each other.
Next, the principle of the system using the optical coupler described in the above publication will be described with reference to Fig. 19. In this system, a signal beam transmitted via a first optical fiber 204 and a reference beam transmitted via a second optical fiber 205 are guided to an optical coupler 207, and the wavefronts of both light beams are aligned with each other by the optical coupler 207.
The above spatial tracking system, however, has problems such as time-consuming operation needed for wavefront alignment of the signal beam 206 and the reference beam 202 and the large-sized structure of the system itself.
Meanwhile, in the above method using the optical coupler 207, the positional accuracy is strictly required for coupling the optical fibers 204 and 205 with the optical coupler 207. However, due to the difficulty in obtaining this positional accuracy, the wavefront alighment is not precisely performed. In addition, the method has a drawback of poor optical coupling rate.
The problems as described above will be summarized as follows:
(a) as for the spatial tracking system, an apparatus for mechenically controlling the wavefronts is required; and a time-consuming wavefront control and a large-sized system structure are also problematic;
(b) as for the method using an optical coupler, the loss of light is remarkable; and an alignment technique for highly accurate positional alignment between the optical coupler and an optical fiber is needed.